1. Field of the Invention
The invention relates to fused ring bridge, ring-locked molecules featuring various donor and acceptor groups or donor- and acceptor-like heterocycles and a stable benzene-, naphthalene- or anthracene-derived polyene bridge, i.e., fused ring bridge, ring-locked compounds, as exemplified below, and methods of their use.
2. Description of the Related Art
The basic elements of photorefractivity include photosensitivity, photoconductivity and electrooptical activity. As an example, the photorefractive (PR) effect enables the recording of optical information in three-dimensional solids through the optical generation of an internal space-charge field and refractive index changes in the solid, as described in P. Gxc3xcnter and J.P. Huignard, Photorefractive Materials and Their Applications, Vol. 1 and 2 (Springer, Berlin, 1988 and 1989), incorporated herein by reference. Due to high optical sensitivity and the ability to erase and rewrite information optically in real-time, photorefractive materials are expected to play a major role in photonic technologies. One special aspect of the PR recording mechanism is the non-zero phase shift that can exist between the optical intensity pattern and the stored phase replica. This so-called nonlocal response can be used to transfer energy between two interacting beams. For some applications, such as image amplification, a strong energy transfer is desired, as described in A. Grunnet-Jepsen, C. L. Thomson, W. E. Moerner, Science 277, 549 (1997). However, for other applications based on the recording and retrieval of stored holograms, the important material parameter is the dynamic range or index modulation An, and one of the current challenges is to increase it.
The photorefractive effect in inorganic crystals has been studied. The mechanism of the refractive index modulation by the internal space-charge field was based on the linear electro-optic effect (i.e., the Pockels effect), as described in A. Ashkin, G. D. Boyd, J. M. Dziedic, R. G. Smith, A. A. Ballmann, and K. Nassau, Appl. Phys. Lett. 9, 72 (1966). More recently, polymers have emerged as new photorefractive materials as described in S. Ducharme, J. C. Scott, R. J. Twieg, and W. E. Moerner, Phys. Rev. Lett. 66, 1846 (1991), W. E. Moerner and S. M. Silence, Chem. Rev. 94, 127 (1994), Y. Zhang, R. Burzynski, S. Ghosal and M. K. Casstevens, Adv. Mater. 8, 111 (1996), B. Kippelen, K. Meerholz, and N. Peyghambarian in Nonlinear Optics of Organic Molecules and Polymers, edited by H. S. Nalwa and S. Miyata (CRC, Boca, Raton, 1997), p. 465, all of which are incorporated herein by reference.
Active organic polymers are emerging as key materials for advanced information and telecommunication technology. Owing to their outstanding performance, structural flexibility, and lightweight, polymers are expected to play a major role in optical technology, especially when large-scale manufacturing of devices at low cost is crucial. Other important characteristics,, that may be desirable depending on the application include sufficiently long shelf life, optical quality, thermal stability and processing ease. Multifunctional nonlinear optical polymers and molecular assemblies are intensively investigated for electrooptic and photorefractive applications.
In addition, with the rapid improvement of the performance of guest/host PR polymer composites, as described in M. C. J. M. Donckers, S. M. Silence, C. A. Walsh, F. Hache, D. M. Burland, and W. E. Moerner, Opt. Lett. 18, 1044 (1993), M. Liphard, A. Goonesekera, B. E. Jones, S. Ducharme, J. M. Takacs., and L. Zhang, Science 263, 367 (1994) and the report of near 100% diffraction efficiency in 105 xcexcm thick samples containing a photoconducting sensitizer by K. Meerholz, B. L. Volodin, Sandalphon, B. Kippelen, and N. Peyghambarian, Nature 371, 497 (1994), it became apparent that the Pockels effect alone could not account for the origin of the high refractive index changes. A detailed explanation of this effect was provided with the orientational enhancement model proposed by W. E. Moerner, S. M. Silence, F. Hache, and G. C. Bjorklund, J. Opt. Soc. Am. B 11, 320 (1994) in which both the birefringence induced by the orientation of the dopant molecules [M. G. Kuzyk, J. E. Sohn, C. W. Dirk, J. Opt. Soc. Am. B 7, 842 (1990), J. W. Wu, J. Opt. Soc. Am. B 8, 142 (1991)] an the molecules"" electrooptic properties were reported to contribute to the refractive index modulation changes.
Thus, on a molecular level, according to the oriented gas model [D. J. Williams in Nonlinear Optical Properties of Organic Molecules and Crystals; Vol. 1, edited by D. S. Chemla and J. Zyss (Academic Press, Inc., Orlando, 1987), K. D. Singer, M. G. Kuzyk, and J. E. Sohn, J. Opt. Soc. Am. B 4, 968 (1987), molecular figure of merit F may be defined for the optimization of the PR effect:
F=A(T)xcexc2xcex94xcex1+xcexcxcex2xe2x80x83xe2x80x83(1) 
where xcex94xcex1 is the polarizability anisotropy of the chromophore, xcexc is its dipole moment, xcex2 is its first hyperpolarizability, and A(T)=2/(9kT) is a scaling factor (kT is thermal energy).
Early PR polymeric compositions were based on dopant molecules (i.e., chromophores) such as 3-fluoro-4-N,N-diethylamino-nitrostyrene (FDEANST) and 2,5-dimethyl-4-p-nitrophenylazoanisole (DMNPAA) because of these dopants"" electro-optic properties. More recent PR polymeric compositions, described in P. M. Lundquist, R. Wortmann, C. Geletneky, R. J. Twieg, M. Jurich, V. Y. Lee, C. R. Moylan, D. M. Burland, Science 274, 1182 (1996) (In this reference, the value for refractive index 0 reported in the Table 2 for a sample with composition 2BNCM:PMMA:TNF 90:10:0.3 wt. % for which four-wave mixing results are presented in FIG. 1, should read 1.5xc3x9710xe2x88x923 instead of 10xc3x9710xe2x88x923) were based on dopants such as N-2-butyl-2,6-dimethyl-4H-pyridone-4-ylidenecyanomethylacetate (2BNCM) with large polarizability anisotropy, improving the dynamic range by a factor of 1.5 over the best previous PR polymeric compositions doped with DMNPAA.
Using model calculations and bond order alternation theory, we have shown recently in B. Kippelen, F. Meyers, N. Peyghambarian, and S. R. Marder, J. Am. Chem. Soc. 119, 4559 (1997) that the orientational birefringence contribution is enhanced for chromophores that are polarized beyond the cyanine limit, i.e., for chromophores that feature both high xcex94xcex1 and high xcexc.
In order to explore this molecular design rationale, the present invention focuses on linear molecules such as polyenes, as opposed to chromophores that contain benzene rings in the bridge. These polyenes exhibit a considerable charge transfer that is confined along the quasi one-dimensional xcfx80-conjugated bridge providing a large xcex94xcex1 and can lead to an important charge separation in the ground-state that provides large molecular dipole moment and polarizability anisotropy, as predicted in B. Kippelen, F. Meyers, N. Peyghambarian, and S. R. Marder, J. Am. Chem. Soc. 119, 4559 (1997). Furthermore, given the known thermal instability of donor-acceptor substituted polymethine bridged chromophore dyes (where the polymethine bridge is formed from CH groups joined by single or double bonds) at elevated temperatures, described below, we discovered the importance of the structure of fused ring bridge, ring-locked dyes according to the invention described herein and exemplified below where the donors and the acceptors induce a large molecular dipole moment and also increase polarizability anisotropy. 
Preliminary work in synthesis of charged fused ring bridge, ring-locked ionic cyanine-like compounds was reported in Y. L. Slominskii, L. M. Shulezho, Ukr. Khim. Zhu., 40, 625-629 (1974); F. S. Babichev, N. N. Romanov, Y. L. Slominskii, A. I. Tolmachev, Ukr. Khim. Zhu.,40, 1165-1170 (1975); Y. L. Slominskii, A. V. Kuleshin, A. I. Tolmachev, Zhu. Org. Khim., 6, 1936-1940 (1970); Y. L. Slominskii, A. L. Skulxe2x80x2bidenko, A. I. Tolmachev Ukr.Khim. Zhu., 40, 1166-1173 (1974), which are incorporated herein by reference. Work on the synthesis of anthracene-like fused ring bridge, ring-locked cyanides has also been reported by Y. L. Slominskii, A. L. Skulxe2x80x2bidenko, A. I. Tolmachev, Zhu. Org. Khim., 11, 392-397 (1975); G. Heilig, W. Lxc3xcttke Chem. Ber., 119, 3102-3108 (1986); G. Heilig, W. Lxc3xcttke, Chem. Ber., 120, 1863-1866 (1987); G. Heilig, W. Lxc3xcttke, Chem. Ber., 121, 407-410 (1988), all of which are incorporated herein by reference.
In contrast, this invention teaches how to synthesize fused ring bridge, ring-locked dyes of the type shown in the General Structures 1, 2 and 3 below, by adapting the chemistry used to synthesize unbridged compounds. Exemplary synthetic routes are described in L. G. S. Brooker (1939), U.S. Pat. No. 2,161,331; L. G. S. Brooker (1939), U.S. Pat. No. 2,170,804; L. G. S. Brooker (1939), U.S. Pat. No. 2,170,807; L. G. S. Brooker (1940), U.S. Pat. No. 2,185,182; L. G. S. Brooker, F. L. White, (1941) U.S. Pat. No. 2,231,659; L. G. S. Brooker, F. L. White (1944), U.S. Pat. No. 2,341,357; L. G. S. Brooker, H. W. J. Cressman (1946), U.S. Pat. No. 2,409,189; L. G. S. Brooker, G. H. Keyes (1948), U.S. Pat. No. 2,441,530; L. G. S. Brooker, G. H. Keyes (1950), U.S. Pat. No. 2,493,748; L. G. S. Brooker, F. L. White (1950), U.S. Pat. No. 2,494,031; L. G. S. Brooker, et al., J. Am. Chem. Soc. 73, 5332-5350 (1951); and L. G. S. Brooker, et al., J. Am. Chem. Soc. 73, 5326-5332 (1951) which are incorporated herein by reference.
The present invention also teaches how the linear polarizability anisotropy and large dipole moment of chromophores according to the invention are particularly useful components in photorefractive compositions operating in the electromagnetic spectrum between 500 and 1600 nm.
Previous work reports on the theory and synthesis of dyes and their second-order nonlinear optical properties, including C. B. Gorman, S. R. Marder, Proc. Natl. Acad. Sci., USA 90, 11297-11301 (1993); H. Ikeda, Y. Kawabe, T. Sakai, K. Kawasaki, Chem. Lett., 1803-1806 (1989); S. R. Marder, D. N. Beratan, L.-T. Cheng, Science 252, 103-106 (1991); F. Meyers, S. R. Marder, B. M. Pierce, J. L. Brxc3xa9das, J. Am. Chem. Soc. 116, 10703 (1994), each of which is incorporated herein by reference. However, in contrast to this previous work, this invention focuses on the photorefractive efficacy of fused ring bridge, ring-locked compounds, whereas the previous papers described dyes having a simple polymethine bridge. More importantly, the effect that leads to the large photorefractive effect in chromophores according to the present invention is not based upon the chromophores"" first hyperpolarizability, but rather on a strategy which we have implemented successfully . As described herein, we have been able to optimize an orientations contribution to the refractive index based upon optimizing the molecular polarizability anisotropy and molecular dipole moment.
Many of the chromophores useful according to the invention can be described by a structural motif where fused ring bridge, ring-locked structures of the form described herein are substituted with donors and acceptors (wherein either the donor or the acceptor or both have a dipole moment greater than 2 Debye) to induce a large molecular dipole moment, and also to increase the molecular polarizability anisotropy. It is a further feature of the invention that the fused ring bridge, ring-locked structure imparts a unusually high thermal stability to the molecules and that also prevents rotation around bonds (e.g., photoisomerization) in the bridge to prohibit changing of the dipole moment or polarizability anisotropy due to the rotation.
Chromophores according to the invention are electrically neutral molecules with desirable orientational and transport properties such that upon illumination of a composition containing the chromophore, the chromophore facilitates charge transport and refractive index modulation.
We have found that molecules which take these forms can be designed to operate in methods wherein the chromophores, alone or as part of a polymeric composition, upon application of a field (either externally applied or internally created in the presence of light) induce or change the angular distribution of the chromophores, resulting in a change in the composition""s refractive index. In particular, chromophores in this invention can be used, even at relatively high loadings, to impart photorefractivity (e.g., to a polymeric composition) without adversely affecting other important properties, such as thermoplasticity or optical transparency. These compositions can be particularly advantageous for fabricating thermally stable, optical quality devices, e.g., by injection molding techniques, since heating of the composition to melt the chromophore followed by quenching to stabilize the chromophore in solution can be avoided. Phase separations observed with other chromophores can be avoided. Thus, photorefractive compositions according to the invention have great utility in a variety of applications, including optical recording and imaging of biological tissues.